Multiple-element sensors (such as, imaging devices) are widely used. It is desirable for a multiple-element sensor to have a wide dynamic range. It is desirable for an X-ray imaging element or an infrared imaging element, in particular, to have a wide dynamic range.
An infrared imaging device that uses an infrared imaging element is characterized by being able to measure temperature in a non-contact manner, and is widely used in various fields (such as, security, medical care, preservation, R&D and military affairs). The infrared imaging device is used for, e.g., measuring a body temperature of a passenger in a non-contact manner at an airport so as to extract a patient having an infectious disease. Related arts are disclosed in, e.g., Japanese Laid-open Patent Publication Nos. 2008-278284, 2008-236158 and 2000-224440. An exemplary infrared imaging device that uses an infrared imaging element will be explained below, although the art described herein is not limited to the example.
FIG. 1 illustrates an exemplary image system that uses an infrared imaging device. As illustrated in FIG. 1, a lens 3 projects an infrared image emitted by an observed object 4 on an infrared imaging element 2 of an infrared imaging device 1. The projected infrared image is converted into an electric signal by a photosensitive portion of the infrared imaging element 2. The electric signal is multiplexed by a reading circuit provided in the infrared imaging element 2, and then sent to a signal processing circuit 10 provided outside the infrared imaging element 2.
The signal processing circuit 10 includes an, e.g., A/D converter circuit 11, an operational circuit 12, a memory 13 and a D/A converter circuit 14. The A/D converter circuit 11 converts an analog signal provided by the reading circuit into a digital signal. The operational circuit 12 carries out gain and offset correction on the digital signal provided by the A/D converter circuit 11. The operational circuit 12 further corrects a variation of every pixel on the basis of correction data stored in the memory 13 so as to produce a corrected digital signal. The corrected digital signal is a final outcome of the A/D conversion. What is called the A/D conversion process includes the process carried out by the operational circuit 12. The D/A converter circuit 14 converts the corrected digital signal into an analog signal and outputs the analog signal. A display 15 displays the infrared image of the observed object 4 on the basis of the analog signal. The corrected digital signal is stored in an external storage device or the like, as necessary.
FIG. 2 illustrates an exemplary infrared imaging element (chip) 2. As illustrated in FIG. 2, the infrared imaging element 2 includes an infrared sensor (photosensitive portion) 5 formed by, e.g., a compound semiconductor material, and a reading circuit unit 6 formed by silicon (Si) material. Mating electrodes of the infrared sensor 5 and the reading circuit unit 6 are coupled to each other by a bump 7 made of indium (In).
An infrared imaging element should preferably be able to capture infrared images of high precision, high sensitivity and low noise at a high-speed frame rate. Thus, the infrared sensor 5 is made of a compound semiconductor of high sensitivity. It is however difficult to make it significantly uniform at present. Hence, the signal processing circuit 10 is provided outside the sensor chip, so that a sensitivity variation, a dark current variation and a nonlinear relationship between an amount of incident rays and an output current of every element in the chip are corrected. The digital operational circuit 12 can thereby be used for carrying out a correcting operation or the like, after the A/D converter circuit 11 A/D-converts the output of the infrared sensor, so that an infrared image of low noise can be obtained.
FIG. 3 illustrates an exemplary reading circuit unit 6. A reading circuit 20 includes, e.g., a plurality of scan lines SL horizontally extended parallel, a plurality of vertical bus lines BL vertically extended parallel, and a plurality of signal input circuits 21 arranged in a matrix corresponding to crossings of the plural scan lines SL and the plural bus lines BL.
A sensor 24 in the signal input circuit 21 indicates one element (cell) in a photosensitive element array provided in the infrared sensor (photosensitive portion) 5. The signal input circuit 21 is provided for every one of the sensors 24 of the photosensitive element array. The sensor 24 is coupled to the signal input circuit 21 provided in the reading circuit unit 6 by an In-bump 7.
A reset signal S2 is applied to a transistor Tr6 of the signal input circuit 21, so that a current flows through the transistor Tr6 and that an integral capacitor C1 is charged up to a particular value. While an integral signal S1 is being applied to a transistor Tr5 after the application of the signal S2 stops, a current corresponding to infrared rays intensity flows through the sensor 24 and a voltage of the transistor Tr5 grows to a voltage corresponding to the infrared rays intensity. Sample/hold (S/L) signals S3 and /S3 are applied to a transfer gate SW1, and the voltage of the integral capacitor C1 is transferred to and held by an S/L capacitor C2. The signal input circuits 21 each carry out such an operation, and a voltage corresponding to the infrared rays intensity of each of the sensors is held by the S/L capacitor C2.
A vertical scan shift register 22 serially outputs scan pulses for choosing the plural scan lines SL one by one. Currents flow in response to the scan pulse through the transistors Tr2 whose gates are coupled to the chosen scan line, and the voltages held by the S/L capacitors C2 of the signal input circuits 21 of the chosen scan line are output to the plural vertical bus lines BL through the transistors Tr1 and Tr2. A horizontal scan register 23 serially applies reading pulses to the transistors Tr3. Voltages on the vertical scan bus lines BL are serially output, via a reading line 26, from a final output stage amplifier 24 to an output line 25 as an analog output signal Vout in response to the reading pulses. After the voltage outputs of the entire vertical scan lines BL end, the vertical scan shift register 22 applies a scan pulse to the next scan line SL. The above operations are repeated since then, so that the signals of all the two-dimensionally arranged sensors 24 are multiplexed and output onto the one output line. The transistor Tr4 is turned on depending upon a signal S4 so as to reset and ground the reading line 26.
Thus, as illustrated in FIG. 4A, a signal of one frame that corresponds to one screen is output in such a way as to form a cluster for every line. If the number of the scan lines SL is N, a signal including N clusters is output. Each of the clusters includes the analog output signals Vout as many as the number of the vertical bus lines BL.
The analog output signal Vout is input to the signal processing circuit 10 and made visual. The A/D converter circuit 11 serially converts the signal of one sensor illustrated in FIG. 4A into a digital signal. As illustrated in FIG. 4B, an amplifier gain, etc., are selected so that the analog output signal Vout changes within an input range R of the A/D converter circuit 11.
FIG. 5 illustrates an exemplary process of the operational circuit 12 of the signal processing circuit 10. The operational circuit 12 identifies which zone of the graph in FIG. 5 (four zones A-D in FIG. 5) a signal voltage of each of the pixels is located in, carries out a correcting operation in reference to offset and gain coefficients provided for every region of the pixels and stored in the memory 13, and finds out a temperature of the object. Such a system for carrying out correction of high accuracy is called a multiple-point correcting system. It is preferable to carry out subtracting processes for every pixel plural times so as to identify the zone of the voltage. It is preferable to carry out multiplying and adding processes for every pixel for the offset and gain correction process. Every pixel and region is provided with one set of the offset and gain coefficients which are stored as a table in the memory 13 (such as, a ROM having a large size). If a highly accurate A/D converter IC is used so that an infrared image of low noise is obtained, words are lengthened in the operational circuit and the memory 13 in which the coefficient table is stored requires a large capacity. In order to mount the highly accurate A/D converter IC on a printed circuit board, it is preferable to separate power supply units for analog and digital circuits from each other so as to prevent noise, and to take measures on wiring patterns for preventing noise caused by a surrounding signal.
In order to obtain a highly precise infrared image, it is preferable to make a pixel format of an infrared sensor as large as 1000 pixels times 1000 pixels, to make an offset and gain coefficient table into a large scale and to work an increase operational circuit that carries out an increasing process within a regular period of time at high speed. Similarly, it is preferable to work an operational circuit at high speed in order to obtain a fast frame rate. In order to provide the infrared device with high performance, the signal processing circuit 10 becomes complicated and is made into a large scale. Thus, it is difficult to implement the infrared device at low cost.
In order to implement a highly precise infrared image or imaging at high speed, it is preferable to A/D-convert an output signal of a large-scale sensor array formed by multiple pixels of infrared imaging elements highly precisely and at a high sampling rate, and to carry out digital signal processing at high speed. There is a problem, however, in that the signal processing circuit 10 that carries out A/D conversion and a correcting operation is made more complicated as the requirement is made to a higher degree, and it becomes difficult to meet the requirement.
As described above, the analog output signal Vout provided by the reading circuit is converted into a digital signal by the A/D converter circuit 11. For instance, a scene having a background temperature around 300K should preferably be made visual with temperature resolution around 0.01K by means of signal processing for an infrared image. The scene having been made visual should preferably be displayed with resolution higher than 15 bits. Taking the signal processing into account, it should preferably be A/D-converted with 16-bit accuracy and at high speed. However, a problem, as described below, exists.
What is ordinarily used for the A/D converter circuit 11 is an A/D converter IC of 14-bit accuracy. An A/D converter IC of 16-bit is quite expensive, and its working speed is limited.
Further, if a high-performance A/D converter IC of 16-bit accuracy is used, ground lines are separated for, e.g., assembly of a printed circuit board (such as, wiring or circuit element arrangements) in order that the highest performance put on a catalog is implemented. In such a case, it is possible for the assembly design to be so difficult that the highest performance cannot be implemented depending upon design conditions.
The operational circuit 12 should preferably carry out an operation process including an addition and a multiplication of 16-bit data with a long word length. A problem arises, however, in that the whole signal processing circuit 10 is rendered very complicated and the cost increases.